Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into harmless gases. When the engine is operated in a stoichiometric or slightly rich air/fuel (A/F) ratio, catalysts containing palladium, platinum, and/or rhodium are able to efficiently convert all three gases simultaneously. That is, the carbon monoxide and hydrocarbons are oxidized to carbon dioxide and water and the NOx is reduced to nitrogen. Hence, such catalysts are often called “three-way” catalysts. It is desirable, however, to operate the engine in a “lean-burn” condition where the A/F ratio is greater than the 14.4-14.7 stoichiometric range, generally between 19 and 27, to realize a benefit in fuel economy. While such precious metal three-way catalysts are able to efficiently convert carbon monoxide and hydrocarbons during lean-burn (excess oxygen) operation, they are not efficient in converting the NOx under lean-burn conditions. Lean-burn, high air-to-fuel ratio, and diesel engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles, the control of NOx emissions from the vehicles continues to post a challenge. Thus, development of an effective and durable catalyst for controlling NOx emissions under net oxidizing conditions accordingly is urgently needed.
Catalysts containing platinum and zeolite are known to be active for NOx reduction by hydrocarbons under lean conditions. However, this catalytic activity is significant only in a narrow temperature range around the light-off temperature of heavy hydrocarbon oxidation, typically between 180° C. and 250° C. Above the light-off temperature, the lean-NOx catalysts quickly lose their catalytic activity because almost all hydrocarbon reductant completely oxidized and not available for NOx reduction. This narrow temperature window of the lean-NOx catalysts is considered to be one of the major technical obstacles, because it makes practical application of these catalysts difficult (for lean-burn gasoline or diesel engines). Base metal containing zeolite catalysts shows activity of NOx reduction by hydrocarbons at higher temperature, typically above 300° C. But they exhibit very little NOx conversion at lower temperatures. In addition, these catalysts deactivate irreversibly if a certain temperature is exceeded. Catalyst deactivation is also found to be accelerated by the presence of water vapor and sulfur containing compound. Thus, it is difficult to consider for commercial use.
Alternative is to employ selective catalytic reduction (SCR) technology using ammonia or urea as a reductant. Vanadium containing and certain zeolite containing catalysts are found to be quite efficient in selective reduction of NOx to N2 in a lean exhaust. Commercial use of SCR technology has been developed for heavy duty diesel application. However, urea supply infra-structure and OBD requirement make SCR technology difficult to apply to all lean burn vehicles. Thus, the art continues to search NOx reduction technology using on-board fuel system.
One effective method to reduce NOx from the exhaust of lean-burn engines, such as gasoline direct injection and partial lean-burn engines, as well as from diesel engines, requires trapping and storing of NOx under lean burn engine operating conditions and reducing the trapped NOx under stoichiometric or rich engine operating conditions or lean engine operating with external fuel injected in the exhaust to induce rich conditions. The lean operating cycle is typically between 1 minute and 20 minutes and the rich operating cycle is typically short (1 to 10 seconds) to preserve as much fuel as possible. To enhance NOx conversion efficiency, the short and frequent regeneration is favored over long but less frequent regeneration. Thus, a lean NOx trap catalyst generally must provide a NOx trapping function and a three-way conversion function.
The lean-NOx-trap technology has been limited to use for low sulfur fuels because catalysts that are active for converting NO to NO2 are also active in converting SO2 to SO3. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxidized conditions (exhaust containing excess oxygen), SOx adsorbs more strongly on NO2 adsorption sites than NO2, and the adsorbed SOx does not desorb under fuel-rich conditions in normal operating conditions. It is found that the surface adsorbed SOx can be removed at high temperature, typically greater than 600° C. under rich (reducing) conditions. Periodic removal of sulfur accumulated on the catalyst tends to rejuvenate the performance and useful life of the lean NOx trap can be prolonged. In new generation of diesel powered vehicles (MY2007 and beyond), many have equipped with a diesel particulate filter (DPF) device to remove the harmful carbonaceous particles. The particulate filter periodic goes through a regeneration cycle, typically around 600° C. to burn off the collected soot. In an exhaust aftertreatment system containing DPF and LNT, it is advantageous to incorporate a sulfur removal event during the regeneration of the particulate filter. Thus, a cost effective sulfur removal process can be easily achieved. In 2007 and beyond, diesel fuel sulfur has been mandated to lower to less than 15 ppm. The advance in engine design, exhaust aftertreatment device and low sulfur fuel makes the lean NOx trap technology attractive for the reduction of NOx emissions from a diesel engine.
Current lean NOx trap (LNT) systems contain alkali and alkaline earth (e.g., Ba) elements. The alkaline earth containing LNT systems show good and durable NOx conversion efficiency between 250° C. and 450° C. However, the LNT exhibits limited NOx conversion below 250° C. It is believed that the presence of trapping components (e.g., Ba) in the LNT catalyst hinders the intrinsic activity of NO oxidation and NOx reduction over the precious metal elements at low temperatures (<250° C.) especially after exposure to high temperature in excess of 750° C. in a lean environment (containing excess oxygen). Since exhaust temperature of a diesel engine under low load and low speed conditions (e.g., FTP75 driving cycle) typically runs below 250° C., it is highly desirable that a lean NOx trap (LNT) catalyst system performs well at such conditions. The novel catalyst systems of the present invention overcome the deficiency of low temperature performance of current lean NOx trap systems.